A scintillator converts X-rays into visible light. High performance scintillators can be made of a crystalline material, such as CsI. The light can be detected by a sensor e.g. comprising arrays of diodes, a photographic plate, and a charge coupled device (CCD) plate. An image formed accordingly is called a radiogram.
In a scintillator two main performance parameters are typically considered, namely light output performance and modulation transfer function.
The modulation transfer function (MTF) is the spatial frequency response of an imaging system or a component. It is the contrast at a given spatial frequency relative to low frequencies. The MTF is preferably as high as possible.
On a radiogram, objects having different sizes and opacity are displayed with different gray-scale values. MTF is responsible for converting contrast values of different-sized objects (object contrast) into contrast intensity levels in the image (image contrast). For general imaging, the relevant details are in a range between 0 and 2 cycles/mm, which demands high MTF values.
In summary, MTF is the capacity of the detector to transfer the modulation of the input signal at a given spatial frequency to its output.
MTF is a useful measure of true or effective resolution, since it accounts for the amount of blur and contrast over a range of spatial frequencies.
In an example a prior art scintillator is grown on a substrate and has a pillar structure. Due to crystallographic limitations the pillars have a cone-like shape at the top. It has been found that often the pillar diameter is bigger at the top of the pillars compared to the bottom (substrate side) thereof. Each pillar is considered to act as sort of a wave guide for the light and helps maintaining the spatial resolution of the image, e.g. on the CCD or diode array detector.
On the scintillator pillars typically a leveling layer is applied in order to cover the top of the (CsI) pillars. It has been found that the leveling layer preferably should not penetrate in between the pillars. It is believed that this causes the decrease of MTF as light can leave the pillars and show spatial cross-talk.
A problem is that when (moist sensitive) scintillating material is exposed to humid air, the scintillator efficiency starts to change and the image quality degrades over the course of time in terms of MTF and the noise level of the image. For this reason scintillating layers have to be covered with a sealing (or barrier) layer. A disadvantage of sealing layers is that upon application thereof it is difficult to maintain the spatial resolution (MTF). Typically on top of the leveling layer a barrier layer is placed. This layer prevents the penetration of moisture toward the leveling layer and scintillator layer to some extent.
The prior art barrier layers are typically not hermitic, thereby causing degradation of the scintillator over time, e.g. in terms of MTF and output performance.
Also barrier layers are of insufficient quality, e.g. in terms of uniform coverage, uniform characteristics, layer thickness, etc. Such is partly due to inherent complexity of the scintillator, e.g. pillar like structure scintillator material.
Also prior art barrier layers are not good enough at elevated temperatures and/or elevated hygroscopicity, especially over time, and not being hermetic, especially not in combination with improved optical characteristics.
Incidentally some documents recite such barrier layers.
For instance, DE 10 2010 041525 A1 recites a scintillator which is stabilized in terms of moisture. In view of the hygroscopic nature of scintillator material the material is protected from moisture by a so-called atomic layer deposition technique. This layer provides a more isotropic covering for high aspect ratio materials that are typically used for scintillators.
Further EP 1 398 648 A2 recites prevention of an interlayer cleavage between a phosphor layer and a moisture-preventing protective layer. In a radiation converting substrate constituted by forming at least a phosphor layer composed of an alkali halide for converting a radiation into light and a light emission activator, and a moisture-preventing protective layer in succession on a radiation-transmitting substrate, the moisture-preventing protective layer is constituted of a first plasma polymerization film formed from a monomer of a silane compound, and a second plasma polymerization film formed from a monomer of a fluorine-containing unsaturated hydrocarbon.
EP 2 453 263 A2 recites an X-ray detector that has a hybrid photoactive layer that is provided between an cathode and a substrate. The hybrid photoactive layer has several scintillators and a bulk heterojunction to perform indirect X-ray conversion by bulk heterojunction that is designed to absorb light in wavelength range of scintillating radiation of scintillators, so as to form electron-hole pairs that are detected electrically. The bulk heterojunction comprises organic semiconductor materials that are soluble and can be deposited via a spraying process. An independent claim is included for method for manufacturing X-ray detector.
And WO2011/065302 A1 recites a support for a roll-shaped scintillator panel; a scintillator panel which can be produced by an uncomplicated process with high productivity and has improved moisture resistance performance; a process for producing the scintillator panel; and a radiation image detector equipped with the scintillator panel. The support for a roll-shaped scintillator panel is characterized by comprising a base material and a metal thin film layer that has a thickness of 1 to 500 nm and is arranged on the base material. The scintillator panel comprises the support and a fluorescent layer arranged on the support, and is characterized in that each of the light-emitting surface and the side surfaces of the fluorescent layer and the side surfaces of the support is covered with a moisture-resistant protective film.
In view of the above problems there is a need for an improved scintillator which overcomes one or more of the above disadvantages, without jeopardizing functionality and advantages.